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Building%20and%20Running%20Modules

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Title: Building%20and%20Running%20Modules


1
Building and Running Modules
  • Ted Baker ? Andy Wang
  • CIS 4930 / COP 5641

2
Setting Up Your Test System
  • For Linux 2.6.x
  • Building modules requires a configured and built
    kernel tree
  • Can obtain one from kernel.org
  • 2.6 modules are linked against object files found
    in the kernel source tree

3
The Hello World Module
  • include ltlinux/init.hgt
  • include ltlinux/module.hgt
  • MODULE_LICENSE(Dual BSD/GPL)
  • static int hello_init(void)
  • printk(KERN_ALERT Hello, world\n)
  • return 0
  • static void hello_exit(void)
  • printk(KERN_ALERT Goodbye, cruel world\n)
  • module_init(hello_init)
  • module_exit(hello_exit)

No main function
4
The Hello World Module
  • include ltlinux/init.hgt
  • include ltlinux/module.hgt
  • MODULE_LICENSE(Dual BSD/GPL)
  • static int hello_init(void)
  • printk(KERN_ALERT Hello, world\n)
  • return 0
  • static void hello_exit(void)
  • printk(KERN_ALERT Goodbye, cruel world\n)
  • module_init(hello_init)
  • module_exit(hello_exit)

Invoked when the module is loaded
5
The Hello World Module
  • include ltlinux/init.hgt
  • include ltlinux/module.hgt
  • MODULE_LICENSE(Dual BSD/GPL)
  • static int hello_init(void)
  • printk(KERN_ALERT Hello, world\n)
  • return 0
  • static void hello_exit(void)
  • printk(KERN_ALERT Goodbye, cruel world\n)
  • module_init(hello_init)
  • module_exit(hello_exit)

Invoked when the module is removed
6
The Hello World Module
  • include ltlinux/init.hgt
  • include ltlinux/module.hgt
  • MODULE_LICENSE(Dual BSD/GPL)
  • static int hello_init(void)
  • printk(KERN_ALERT Hello, world\n)
  • return 0
  • static void hello_exit(void)
  • printk(KERN_ALERT Goodbye, cruel world\n)
  • module_init(hello_init)
  • module_exit(hello_exit)

Micros to indicate which module initialization
and exit functions to call
7
The Hello World Module
  • include ltlinux/init.hgt
  • include ltlinux/module.hgt
  • MODULE_LICENSE(Dual BSD/GPL)
  • static int hello_init(void)
  • printk(KERN_ALERT Hello, world\n)
  • return 0
  • static void hello_exit(void)
  • printk(KERN_ALERT Goodbye, cruel world\n)
  • module_init(hello_init)
  • module_exit(hello_exit)

This module bears a free license
8
The Hello World Module
  • include ltlinux/init.hgt
  • include ltlinux/module.hgt
  • MODULE_LICENSE(Dual BSD/GPL)
  • static int hello_init(void)
  • printk(KERN_ALERT Hello, world\n)
  • return 0
  • static void hello_exit(void)
  • printk(KERN_ALERT Goodbye, cruel world\n)
  • module_init(hello_init)
  • module_exit(hello_exit)

The ordering matters sometimes
9
The Hello World Module
  • include ltlinux/init.hgt
  • include ltlinux/module.hgt
  • MODULE_LICENSE(Dual BSD/GPL)
  • static int hello_init(void)
  • printk(KERN_ALERT Hello, world\n)
  • return 0
  • static void hello_exit(void)
  • printk(KERN_ALERT Goodbye, cruel world\n)
  • module_init(hello_init)
  • module_exit(hello_exit)

printf in C library No floating-point
support
10
The Hello World Module
  • include ltlinux/init.hgt
  • include ltlinux/module.hgt
  • MODULE_LICENSE(Dual BSD/GPL)
  • static int hello_init(void)
  • printk(KERN_ALERT Hello, world\n)
  • return 0
  • static void hello_exit(void)
  • printk(KERN_ALERT Goodbye, cruel world\n)
  • module_init(hello_init)
  • module_exit(hello_exit)

Indicates the message priority Note that no ,
after KERN_ALERT
11
Module Loading/Unloading
  • make C /home/awang/linux-2.6.25.3 Mpwd
    modules

Notice the quote
12
Module Loading/Unloading
  • make C /home/awang/linux-2.6.25.3 Mpwd
    modules
  • make1 Entering directory /usr/src/linux-2.6.25
    .3
  • CC M /home/awang/hello/hello.o
  • Building modules, stage 2.
  • MODPOST
  • CC /home/awang/hello/hello.mod.o
  • LD M /home/awang/hello/hello.ko
  • make1 Leaving directory /usr/src/linux-2.6.25.
    3

13
Module Loading/Unloading
  • make C /home/awang/linux-2.6.25.3 Mpwd
    modules
  • make1 Entering directory /usr/src/linux-2.6.25
    .3
  • CC M /home/awang/hello/hello.o
  • Building modules, stage 2.
  • MODPOST
  • CC /home/awang/hello/hello.mod.o
  • LD M /home/awang/hello/hello.ko
  • make1 Leaving directory /usr/src/linux-2.6.25.
    3
  • su
  • Password

14
Module Loading/Unloading
  • make C /home/awang/linux-2.6.25.3 Mpwd
    modules
  • make1 Entering directory /usr/src/linux-2.6.25
    .3
  • CC M /home/awang/hello/hello.o
  • Building modules, stage 2.
  • MODPOST
  • CC /home/awang/hello/hello.mod.o
  • LD M /home/awang/hello/hello.ko
  • make1 Leaving directory /usr/src/linux-2.6.25.
    3
  • su
  • Password
  • root

15
Module Loading/Unloading
  • make C /home/awang/linux-2.6.25.3 Mpwd
    modules
  • make1 Entering directory /usr/src/linux-2.6.25
    .3
  • CC M /home/awang/hello/hello.o
  • Building modules, stage 2.
  • MODPOST
  • CC /home/awang/hello/hello.mod.o
  • LD M /home/awang/hello/hello.ko
  • make1 Leaving directory /usr/src/linux-2.6.25.
    3
  • su
  • Password
  • root /sbin/insmod ./module.ko

16
Module Loading/Unloading
  • make C /home/awang/linux-2.6.25.3 Mpwd
    modules
  • make1 Entering directory /usr/src/linux-2.6.25
    .3
  • CC M /home/awang/hello/hello.o
  • Building modules, stage 2.
  • MODPOST
  • CC /home/awang/hello/hello.mod.o
  • LD M /home/awang/hello/hello.ko
  • make1 Leaving directory /usr/src/linux-2.6.25.
    3
  • su
  • Password
  • root /sbin/insmod ./module.ko
  • Hello, world
  • root

Might be printed to /var/log/messages
17
Module Loading/Unloading
  • make C /home/awang/linux-2.6.25.3 Mpwd
    modules
  • make1 Entering directory /usr/src/linux-2.6.25
    .3
  • CC M /home/awang/hello/hello.o
  • Building modules, stage 2.
  • MODPOST
  • CC /home/awang/hello/hello.mod.o
  • LD M /home/awang/hello/hello.ko
  • make1 Leaving directory /usr/src/linux-2.6.25.
    3
  • su
  • Password
  • root /sbin/insmod ./module.ko
  • Hello, world
  • root /sbin/rmmod module.ko

Either module or module.ko
18
Module Loading/Unloading
  • make C /home/awang/linux-2.6.25.3 Mpwd
    modules
  • make1 Entering directory /usr/src/linux-2.6.25
    .3
  • CC M /home/awang/hello/hello.o
  • Building modules, stage 2.
  • MODPOST
  • CC /home/awang/hello/hello.mod.o
  • LD M /home/awang/hello/hello.ko
  • make1 Leaving directory /usr/src/linux-2.6.25.
    3
  • su
  • Password
  • root /sbin/insmod ./module.ko
  • Hello, world
  • root /sbin/rmmod module.ko
  • Goodbye cruel world
  • root

Might be printed to /var/log/messages
19
Kernel Modules vs. Applications
  • Applications
  • Can access various functions in user-level
    libraries (e.g., printf in C library)
  • Kernel modules
  • No user-level libraries
  • printk is defined within the kernel
  • Exported to modules
  • Should include only header files defined within
    the kernel source tree

20
Linking a Module to the Kernel
21
Threads/Processes
  • Thread A sequential execution stream
  • Address space Chunks of memory and everything
    needed to run a program
  • Process An address space thread(s)

22
User Space and Kernel Space
  • Kernel modules run in kernel space
  • Execute in the supervisor mode
  • Everything is allowed
  • Share the same address space
  • Applications run in user space
  • Execute in the user mode
  • Restricted access to hardware
  • Each has its own address space

23
System Calls
  • System calls allow processes running at the user
    mode to access kernel functions that run under
    the kernel mode
  • Prevent processes from doing bad things, such as
  • Halting the entire operating system
  • Modifying the MBR

24
Hardware Interrupts
  • Can suspend user-level processes
  • Transfers execution from user space to kernel
    space
  • Interrupts are handled by separate threads
  • Not related to any user-level processes
  • Asynchronous

25
Role of a Module
  • Extend kernel functionality
  • Modularized code running in kernel space

26
Concurrency in the Kernel
  • Sources of concurrency
  • Hardware interrupts
  • Kernel timers
  • Multiple CPUs
  • Preemption

27
Handling Concurrency
  • Kernel code needs to be reentrant
  • Capable of running in more than one thread
    execution context at the time
  • Prevent corruption of shared data
  • Avoid race conditions
  • Results depend on the timing of their executions

28
The Current Process
  • Most actions performed by the kernel are done on
    behalf of a specific process
  • The current process
  • Defined as a per CPU MACRO
  • struct task_struct current
  • include ltasm/current.hgt
  • include ltlinux/sched.hgt

29
The Current Process
  • Print the current command name, process ID, and
    task (thread) ID
  • include ltlinux/sched.hgt
  • printk(KERN_INFO The process is \s\ (tgid i)
    (pid i)\n, current-gtcomm,
  • current-gttgid, current-gtpid)

30
A Few Other Details
  • Limited address space for kernel
  • Should dynamically allocate and deallocate space
    for large data structures
  • Functions starting with __ should be used with
    caution
  • Kernel does not support floating point arithmetic

31
Compiling Modules
  • Details on compiling the kernel
  • Documentation/kbuild
  • Required tools with matching versions
  • Compiler, module utilities, and so on...
  • If the version is too new can cause problems as
    well
  • Documentation/Changes

32
Simplest Makefile
  • obj-m hello.o
  • One module to be built from hello.o
  • Resulting module is hello.ko

33
More on Makefiles
  • Suppose you have a module called module.ko
  • Generated from file1.c and file2.c
  • obj-m module.o
  • module-objs file1.o file2.o

34
More on Makefiles
  • To make, type the following in the directory
    containing the module source and Makefile
  • make C /home/awang/linux-2.6.25.3 Mpwd
    modules

Changing to the kernel source directory
35
More on Makefiles
  • To make, type the following in the directory
    containing the module source and Makefile
  • make C /home/awang/linux-2.6.25.3 Mpwd
    modules

Move back to the module source directory
36
A More Elaborate Makefile
  • If KERNELRELEASE is defined, weve been invoked
    from the
  • kernel build system and can use its language
  • ifneq ((KERNELRELEASE),)
  • obj-m hello.o
  • Otherwise we were called directly from the
    command
  • line invoke the kernel build system.
  • else
  • KERNELDIR ? /lib/modules/(shell uname
    r)/build
  • PWD (shell pwd)
  • modules
  • (MAKE) C (KERNELDIR) M(PWD) modules
  • clean
  • rm fr .o core ..cmd .ko .mod.c
    .tmp_versions
  • endif

If KERNELDIR is not defined, define it.
Kernel release version
37
Loading/Unloading Modules
  • insmod
  • Links unresolved symbol in the module to the
    symbol table of the kernel
  • more /proc/modules to see a list of currently
    loaded modules
  • rmmod
  • Removes a kernel module

38
Loading/Unloading Modules
  • rmmod
  • Removes a kernel module
  • Fails when the kernel believes that it is still
    in use
  • Or, something has gone wrong
  • Might need to reboot to remove the module

39
Version Dependency
  • Modules code has to be recompiled for each
    version of the kernel
  • Sensitive to kernel version, compiler version,
    and various configuration variables
  • If things dont match
  • root /sbin/insmod hello.ko
  • Error inserting ./hello.ko -1 Invalid module
    format

40
Version Dependency
  • Possible remedies
  • Check /var/log/messages for specific causes
  • Change KERNELDIR as needed

41
The Kernel Symbol Table
  • Addresses of global functions and variables
  • A module can export its symbols for other modules
    to use
  • Module stacking
  • E.g., MSDOS file system relies on symbols
    exported by the FAT module

42
Module Stacking Example
  • Stacking of parallel port driver modules
  • Can use modprobe to load all modules required by
    a particular module

43
Auto-loading
  • Modify /etc/modprobe.conf
  • Example
  • alias eth0 e1000
  • Whenever eth0 is referenced, the kernel module
    e1000 is loaded

44
Export Module Symbols
  • In module header files
  • Use the following macros
  • EXPORT_SYMBOL(name)
  • EXPORT_SYMBOL_GPL(name)
  • _GPL makes the symbol available only to
    GPL-licensed modules

45
Defending against Namespace Problems
  • Declare all functions and global variables static
    unless you mean to export them
  • Use a module-unique prefix for all exported
    symbols

46
Preliminaries
  • Just about all module code includes the following
    header files
  • ltlinux/module.hgt
  • Symbols and functions needed by modules
  • ltlinux/init.hgt
  • Allows you to specify initialization and cleanup
    functions

47
Initialization and Shutdown
  • Initialization function
  • Registers any facility, or functionality offered
    by the module
  • static int __init initialization_function(void)
  • / initialization code here /
  • module_init(initialization_function)

48
Initialization and Shutdown
  • Initialization function
  • Registers any facility, or functionality offered
    by the module
  • static int __init initialization_function(void)
  • / initialization code here /
  • module_init(initialization_function)

Indicates that the module loader can drop this
function after the module is loaded, making its
memory available
49
Initialization and Shutdown
  • Initialization function
  • Registers any facility, or functionality offered
    by the module
  • static int __init initialization_function(void)
  • / initialization code here /
  • module_init(initialization_function)

Mandatory to specify the initialization function
50
The Cleanup Function
  • Unregisters various functionalities and returns
    all resources
  • static void __exit cleanup_function(void)
  • / Cleanup code here /
  • module_exit(cleanup_function)

51
The Cleanup Function
  • Unregisters various functionalities and returns
    all resources
  • static void __exit cleanup_function(void)
  • / Cleanup code here /
  • module_exit(cleanup_function)

Indicates that this function is for unloading only
52
The Cleanup Function
  • Unregisters various functionalities and returns
    all resources
  • static void __exit cleanup_function(void)
  • / Cleanup code here /
  • module_exit(cleanup_function)

Needed to specify the cleanup function
53
Error Handling During Initialization
  • static int __init my_init_function(void)
  • int err
  • / registration takes a pointer and a name /
  • err register_this(ptr1, skull)
  • if (err) goto fail_this
  • err register_that(ptr2, skull)
  • if (err) goto fail_that
  • err register_those(ptr3, skull)
  • if (err) goto fail_those
  • return 0 / success /
  • fail_those unregister_that(ptr2, skull)
  • fail_that unregister_this(ptr1, skull)
  • fail_this return err / propagate the error
    /

54
Error Handling During Initialization
  • static int __init my_init_function(void)
  • int err
  • / registration takes a pointer and a name /
  • err register_this(ptr1, skull)
  • if (err) goto fail_this
  • err register_that(ptr2, skull)
  • if (err) goto fail_that
  • err register_those(ptr3, skull)
  • if (err) goto fail_those
  • return 0 / success /
  • fail_those unregister_that(ptr2, skull)
  • fail_that unregister_this(ptr1, skull)
  • fail_this return err / propagate the error
    /

Check ltlinux/errno.hgt for error codes
55
Cleanup Function
  • static void __exit my_cleanup_function(void)
  • unregister_those(ptr3, skull)
  • unregister_that(ptr2, skull)
  • unregister_this(ptr1, skull)
  • return err

56
Other Code Patterns
  • int __init my_init(void)
  • int err -ENOMEM
  • item1 allocate_thing(arg1)
  • item2 allocate_thing2(arg2)
  • if (!item1 !item2) goto fail
  • err register_stuff(item1, item2)
  • if (!err)
  • stuff_ok 1
  • else
  • goto fail
  • return 0
  • fail
  • my_cleanup()
  • return err

57
Other Code Patterns
  • void my_cleanup(void)
  • if (item1) release_thing(item1)
  • if (item2) release_thing2(item2)
  • if (stuff_ok) unregister_stuff()
  • return
  • No __exit when it is called by nonexit code

58
Module-Loading Races
  • A facility is available once a register call is
    completed
  • Kernel can make calls to registered functions
    before the initialization function completes
  • Obtain and initialize all critical resources
    before calling the register function

59
Module Parameters
  • Include moduleparam.h, stat.h
  • Need to use the following macros
  • module_param(name, type, permission)
  • module_param_array(name, type, num, permission)

60
Example Use of Module Parameters
  • Allow the hello world module to say hello to
    someone a number of times
  • /sbin/insmod ./hello.ko someoneMom times2
  • Hello Mom
  • Hello Mom

61
Example Use of Module Parameters
  • Need to use the module_param macro
  • static char someone world
  • static int times 1
  • module_param(times, int, S_IRUGO)
  • module_param(someone, charp, S_IRUGO)

Read-only flag, defined in stat.h
62
Support Parameter Types
  • bool
  • charp
  • Memory allocated for user provide strings
  • int, long, short, uint, ulong, ushort
  • Basic integers

63
User Level Facilities
  • X server
  • Some USB drivers
  • Various daemons/threads

64
User Level Facilities
  • Fast development
  • C library support
  • Conventional debugger
  • Fault isolation
  • Portability
  • Interrupts not available
  • Privileged access required for direct memory
    access
  • Poor performance
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